The If any argument is a float and there are fractional microseconds, the fractional microseconds left over from all arguments are combined and their sum is rounded to the nearest microsecond.
If no argument is a float, the conversion and normalization processes are exact (no information is lost).
If the normalized value of days lies outside the indicated range, object represents a date (year, month and day) in an idealized calendar, the current Gregorian calendar indefinitely extended in both directions.
January 1 of year 1 is called day number 1, January 2 of year 1 is called day number 2, and so on.
This matches the definition of the “proleptic Gregorian” calendar in Dershowitz and Reingold’s book Calendrical Calculations, where it’s the base calendar for all computations.